Rain sensors are commonplace for detecting the presence of rain on occasions where such detection is desired. However, most rain sensors rely on moving mechanisms and/or active electronic components, which have performance limitations or, in some cases, may be prohibitively expensive to manufacture and operate.
A number of rain sensors are currently used for irrigation control. One such example, described in U.S. Pat. No. 4,919,165, has a container for sensing a preset rainfall. Disadvantageously, this rain sensor is unable to distinguish between the preset rainfall and heavy rainfall. The sensor returns to a dry state quickly in sunlight, causing the irrigation controller to begin watering prematurely if the rainfall was heavy.
Another example is the Bosch vehicle windshield rain sensor (Optical Sensor U.S. Pat. No. 6,376,824 to Michenfelder et al) used to operate windshield wipers. This sensor depends on the change in refraction of a reflected light beam against glass when water is on the outer glass surface. Natural light, such as sunlight, can trigger false indications when aligned with the sensor field of view.
Another example of a rain sensor is described in U.S. Pat. No. 5,505,082 and uses a turbine wheel rotated by passing funneled water drops over it, and requires replaceable filters to prevent jamming the mechanism.
In another example, U.S. Pat. No. 4,578,995 describes a rain sensor which requires an active electronic circuit to detect a change when raindrops bridge two parallel rods. Similarly, the rain sensor described in U.S. Pat. No. 4,520,667 passes funneled raindrops over two vertically spaced grids and uses a complicated electronic circuit to count drops and display results.
In another example, U.S. Pat. No. 4,305,280 determines rainfall rates by using active electronics to measuring the power required to evaporate rainfall water as it arrives.
Thus, there is a need for an improved, yet simplified rain intensity sensor which addresses one or more of the above-mentioned shortcomings.